Proton spectrum acquisition

Figure n.8 Molecular structure and one-dimensional H MAS spectra of [syr)-(=SiO)Mo(=NAr) (=CHtBu) (CHjtBu)]. (a) Single-pulse spectrum (b) delayed-acquisition spectrum (c) constant-time proton spectrum and (d) constant-time proton spectrum of the... 

Figure 5.3 GHMBC spectrum of a colored impurity formed during the synthesis of Tipranavir. The long-range delay in the experiment was optimized for 10 Hz the data were acquired in 12.5 h. Chemical shift labels show the chemical shift of the carbon to which a given proton is long-range coupled. As can be seen by simple inspection, there was considerable degradation of the sample during the course of the data acquisition as there are peaks in the contour plot corresponding to responses that were not observed in the proton spectrum taken at the outset of data acquisition, which is plotted above the contour plot.

Fig. 4. (a) 300 MHz proton spectrum and (b)-(e) selective reverse INEPT spectra of 28% menthone (Aldrich) in acetone-ds, measured using a 5 mm sample in the 10 mm broadband probe of a Varian Associates XL300 spectrometer using the sequence of fig. 1. The sample contains substantial quantities of isomenthone, seen clearly in the methyl region of trace (a). Spectra (b) to (e) used selective excitation of carbon sites 6, 7, 2 and 8, respectively, with delays 2r of 3.85, 3.85, 1.92 and 1.54 ms. 32 transients were used for each trace no spin lock pulses or 180° pulses were used. Traces (b) to (e) have a vertical scale lOOOx that of trace (a). No homodecoupling was used during acquisition. 

We now consider multiple-pulse experiments and two-dimensional (2-D) NMR. Exactly what does the term dimension in NMR mean The familiar proton spectrum is a plot of frequency (in S units) versus intensity (arbitrary units)—obviously 2-D but called a 1-D NMR experiment, the one-dimension referring to the frequency axis. It is important to remember that the frequency axis, with which we are comfortable, is derived from the time axis (the acquisition time) of the FID through the mathematical process of Fourier transformation. Thus, experimentally, the variable of the abscissa of a 1-D experiment is in time units. 

The refocusing delay just before acquisition has been eliminated, so the peaks in F2 will be antiphase doublets separated by the long-range/cH (2-15 Hz). This splitting is in addition to any H splitting pattern already present in the ID proton spectrum (Fig. 11.11). The second 13 C 90° pulse is phase cycled as before to make sure that only H magnetization... 

The most basic NMR experiment is the one-pulse proton experiment.23-25 Proton chemical shifts typically range from 0 to 10 ppm, so the spectral width should be set at least this large. A good approach is to set the spectral width to a larger value, such as 15 ppm, to identify the actual limits of the resonances observed for a given sample. Then the spectral width can be reset to a smaller value specific to the sample. Acquisition parameter values determined for the 1-D proton spectrum can be used as a guideline for other proton-detected experiments, including the proton dimension of two-dimensional experiments. 

Because of the inherently poor sensitivity, the acquisition of a carbon spectrum should be considered optional. Deciding whether to collect carbon data depends on the expected sensitivity based on the proton spectrum, the anticipated sample stability over the duration of the experiment, the required degree of carbon resolution necessary, and the importance of directly detecting quaternary carbons. Alternatively, the carbon resonances can be... 

Scenario (a) transplants acquisition parameters from a typical ID proton spectrum into the second dimension leading to unacceptable time requirements, whereas (b) and (c) use parameters more appropriate to 2D acquisitions. All calculations use phase cycles for f quad-detection and axial peak suppression only and, for (b) and (c), a recovery delay of Is between scans. A single zero-filling in f] was also employed for (b) and (c). 

Figure 6.8. A comparison of signal suppression methods used in proton-detected heteronuclear correlation experiments (see descriptions in text). Spectrum (a) is taken from a conventional ID proton spectrum without suppression of the parent resonance and displays the required satellites. Other spectra are recorded with (b) phase-cycling, (c) optimised BIRD presaturation, and (d) pulsed field gradients to remove the parent line. All spectra were recorded under otherwise identical acquisition conditions and result from two transients. Complete suppression can be achieved with gradient selection, but at some cost in sensitivity in this case (see text).

Figure 6.23. The selective observation of protons bound to a carbon-13 label (2- C-glycine) with a gradient selected ID HMQC sequence, (a) The ID proton spectrum and the filtered spectrum recorded (b) without and (c) with carbon-13 decoupling during acquisition.

Figure 6.1 reveals a number of interesting features. We note that there is a separate line for the H channel and one for the 13C channel . These channels represent the hardware associated with the irradiation and signal acquisition of each relevant nucleus in our experiments. Following equilibration (td), the pulse sequence used to obtain a 1-D proton spectrum consists of a tt/2x pulse, delay, and signal acquisition of the order of seconds (Fig 6.1a). We also notice that the 13C channel is inactive during a simple proton experiment. Normally, we will not show a given channel unless there is some activity in that channel. 

Figure 2. 2D J-resolved proton spectrum of the protein bovine pancreatic trypsin inhibitor (BPTI). a. High-field region from 0.4—1.6 ppm, which contains the resonances of 19 methyl groups of the 360-MHz H NMR spectra of a 0.01 M solution of BPTI in D O at pH 4.5, 60°C. Prior to the Fourier transformation, the 2D data set was weighted in the t, and ts directions by weighting functions cos[(t.J 2Tx)ir]exp(tx/0.4Tx), with x = 1,2 Tj = 2.46 s, and Ts = 1.23 s are the maximum acquisition times in the ti and tj domains. The 2D J-resolved spectrum was computed from 64 X SI 92 data points and is presented as a (J, spectrum the top trace shows the conventional ID spectrum the bottom trace shows the projection of the 2D spectrum with 4> = rtl4. b. Presentation of the 2D J-resolved H spectrum (a) by cross sections. The resolved multiplets of 19 methyl protons are shown. The 2D resolved spectrum allows the analysis of otherwise overlapping multiplets, the accurate measurement of coupling constants, and the assignment of the resonances. (Reproduced, with permission, from Ref. 14. Copyright 1978, Academic...

There are a number of approaches to generate suitable template multiplets, the simplest of which is to take these from a ID proton spectrum recorded with a delay equal to Alr immediately after the excitation pulse (Fig. 6.44a) so as to precisely simulate the phase evolution occurring in the corresponding HMBC delay. Experimentally, this can in fact be achieved by simply collecting a standard proton acquisition and removing the first part of the FID equivalent to Alr by left-shifting the FID data points. Figure 6.44b shows an optimised... 

Figure 7.18. Traces from the 2D absorption-mode 7-resolved spectrum of menthol 7.1. (a) A region from the ID proton spectrum, (b) the 02 projection of the titled 2D spectrum showing proton-decoupled resonances and (c) the corresponding traces through the/i multiplets in the 2D spectrum. The multiplets for protons 3 and 4 are not fully decoupled for reasons described in the text. The data were acquired with a total acquisition time of 1 s in both t2 and fi using a 60Hz/i window. The selective 180° pulse was a 50 ms Q3 Gaussian cascade, and the gradient strength was 1% of the maximum 53 G cm 0 The broadband 180° pulses were BIPs (720.50.20) applied for 100 Xs at a Bi field strength of 20 kHz.

Typically, H and NMR spectroscopy have been apphed when studying the structure of hb polymers. Although the proton spectrum is the easier to acquire, it is often less informative than the carbon spectrum. In the case of monomers based on amines, sihcon, or phosphorus, valuable information has been obtained from [78, 79], Si [76, 80-88], and NMR [89] spectra. The acquisition of a NMR spectrum should be considered if fluorine is present in the subunits [56,... 

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